I am creating a waterfall and I want to have the fluid physics loop over a set of frames
Your best bet would just be to render out a 10 second sequence (ie. 8 seconds plus a little extra), and then cross fade the clips in a video editor. I don't think you're going to be able to actually loop the fluid sim files with any sort of precision.

You can physically duplicate the fluid sim files that you've cached. If your cache folder contained: fsim1.bobj, fsim2.bobj, fsim3.bobj, then you duplicated them and renamed them fsim1.bob4, fsim5.bobj, fsim6.bobj, then you would then have a 6 frame simulation with the original simulation repeating twice. Figuring out how to blend them would be the difficult part.